Pigment producers offer tin oxide-rich conductive pigments that are very light in color, usually slightly tawny, grayish or off-white colorations for a variety of antistatic uses. Light-colored pigments are desirable for formulating light-colored coatings.
Frequently the tin oxide is doped with a minor proportion of an antimony oxide, indium oxide, phosphoric anhydride, tellurium oxide, germanium oxide, lanthanum oxide, and/or titanium dioxide. U.S. Pat. No. 4,655,966 to Guillaumon et al. shows such white to off-white doped pigments and their incorporation into binders such as solvent-thinned epoxy resins, etc. to make paints.
Composite, desirably light-colored, tin oxide-rich conductive pigments also have taken the form of tin oxide (SnO.sub.2) deposited substantially uniformly on mica flake (e.g. "Meta-Mica", a trademark of Sanyo Color Works, Ltd., Japan), submicron spherical mixed metal oxide powders, e.g. ("Stanostat" powders, Stanostat being the trademark of Keeling & Walker, Ltd., of the U.K.), and antimony oxide-d oxide-coated titanium dioxide pigments such as that of U.S. Pat. No. 4,373,013.
One important use of antistatic coatings of the sort referred to is for floors. Antistatic floor coatings are particularly useful in settings such as factories, "clean" rooms, laboratories, hospital operating rooms, etc. Specifications defining resistance characteristics of antistatic floor coatings have been established over the years. For cured antistatic floor coatings earlier specifications often called for an electrical resistance as low as 25,000-100,000 ohms to an upper value of 1,000,000 ohms. More recently floor specifications have called for resistances from a million to a billion ohms. In this application non-conductive (insulating) surfaces of will be regarded, for simplicity, to be those having electrical surface resistance that is substantially above a billion ohms. The test method used to measure electrical surface resistance here is known as NFPA-99. Another method that may be used is the ASTM Test Method F150-89.
Durable opaque antistatic floor coatings present special challenges. These coatings are typically fairly thick, often 10 mils or more, thus demanding a comparatively large amount of coating per unit area. The coating must contain sufficient antistatic pigment to establish the necessary electrical pathways in the coating and the coating must be opaque. At the same time a glossy floor finish is quite frequently required. As the quantity of pigment and opacifier in a coating increases, the gloss of the cured coat decreases. Accordingly the demand for glossy finishes has tended to limit the amount of pigmentary solids, including antistatic pigment and opacifiers which can be employed.
Antistatic floor coatings have been proposed which contain conductive materials other than tin oxide pigments. For example, conductive carbon-filled or graphite-filled coatings have been proposed which are substantially less expensive than tin oxide pigment coatings for antistatic service. The carbon-filled coatings produce finishes having quite dark tones which have been often deemed undesirable and objectionable for floors.
The cost of desirably light-colored tin oxide-rich conductive pigments generally is quite high in comparison to common coating ingredients. Accordingly, there has been as yet limited acceptance of antistatic coatings containing that sort of conductive pigment, particularly for use on floors, where considerable amounts of the pigment are required. A tin oxide-rich conductive pigment can be regarded as one that has at least about 5% by weight tin oxide, including any doping oxides.
Hard, impalpable achromatic filler minerals have been used in floor coatings for producing hard finishes. These minerals are not characterized as being conductive and have not been used as pigments in the oxide-rich antistatic floor coatings. Accordingly hard, impalpable achromatic filler minerals have not been considered as pigmentary materials in antistatic floor coatings.
It has been discovered that an improved antistatic pigment is provided by blending tin oxide-rich pigment with a hard, impalpable achromatic filler mineral. When this is done the conductivity of the coating is increased substantially over what it would have been had the hard, impalpable achromatic filler mineral not been added. This enables use of reduced quantities of tin oxide pigment. Moreover, the degree of glossiness of the coating is increased over what it would have been had its conductivity level been created by unblended tin-oxide pigment. Advantages of using the instant invention over prior proposals thus include the opportunity for increased facility in formulating and for substantial cost saving while achieving like or better antistatic coating performance (lower surface resistance) with opaque coatings containing tin oxide-rich pigments, and that in a wide range of colors. Surprisingly, this is accomplished by the addition of filler that is a relatively poor conductor of electricity.
The invention also offers the ability to lay down a guide coat that will not increase and can even lower the electrical resistance of an antistatic topcoat that contains a comparatively expensive tin oxide-rich pigment while at the same time to guard against the leaving of inadvertent overly-thin patches or even skimping by the applier.